Question

The string of pendulum of length $l$ is displaced through $90 ^ { \circ }$ from the vertical and released. Then the minimum strength of the string in order to withstand the tension, as the pendulum passes through the mean position is

• A. $m g$
• B. $3 m g$
• C. 5 mg
• D. 6 mg

Answer 1

Answer: (B)

$3 m g$

Answer 2

$T = m g + \frac { m v ^ { 2 } } { l }$ $= m g + 2 m g = 3 m g$

where $v = \sqrt { 2 g l }$ from $\frac { 1 } { 2 } m v ^ { 2 } = m g l$